Microsoft's unsolicited bid for Yahoo is far from a done deal, an analyst cautioned.

In a research note Friday, Bank of America Securities analyst Brian Pitz said that the European Union regulators could cite "potential non-search influence" in trying to block the acquisition.

Mr. Pitz also noted that Yahoo has a "poison pill" in place to block a deal if the board feels Microsoft's $44.6 billion offer is too low.

In his research note, Mr. Pitz said that Google's pending acquisition of Doubleclick plus the failure of Yahoo to perform up to expectations put pressure on Microsoft to move to consummate a deal.

Should a deal be completed, a Microsoft/Yahoo combination would make a "formidable competitor" to Google, which "has been winning the competitive battle with Microsoft in online services," he said.

"Poison pills" are used by corporations to thwart hostile takeovers by making their stock less appealing.

One type of "poison pill," for instance, lets existing shareholders, but not the acquirer, buy more shares at a discount.

Such a strategy can make the price of the target company prohibitively expensive.